Capacitor microphonic testing apparatus



ug. 26, 1952 G. o. sMlTH 2,608,091

cAPAcrToR MICRQPHONIC TESTING APPARATUS Filed sept. s. 1949 PatentedAug. y26, 1952 CAPACITOR MICROPHONIC TESTING A APPARATUS George 0.Smith, Philadelphia, Pa., assigner to Philco Corporation, Philadelphia,Pa., a corporation of Pennsylvania Application September 3, 1949,SerialNo. 113,991

7 Claims.

The present invention relates broadly to condenser testing apparatus,and more particularly, to testing apparatus adapted to determine themicrophonc response of condensers.

Since a condenser (or capacitor) consists of a plurality ofdielectrically spaced conductors and since its capacitance is a functionof this spacing, it is evident that vibration imparted to theseconductors will produce a corresponding variation in the capacitance ofthe condenser. In certain circuit arrangements vibration of thecondenser elements or assemblies may produce highly deleterious effectson the electrical operation of the circuit. Thus if the condenser is,for example, used .as the tuning element of an amplitudemodulationreceiver, vibration of the condenser plates will cause the receivertuning to fluctuate at the frequency of vibration of the condenserplates. If the gain of the receiver is not uniform across thepass-band-and usually it is notthere will result a correspondingamplitude variation of the received signal which is detected along withthe desired modulation and hence issues as sound from the loudspeaker.The sound Waves thus developed impinge on the condenser plates, settingthem again into vibration, thus completing a feedback path which mayfinally produce a spurious sound output due to mechanical condenservibration which is well in excess of the desired sound output. Thisphenomenon is commonly referred to as being due to condensermicrophonics.

In frequency-modulation receivers, vibration f the plates of thelocal-oscillator section of the gang condenser is especially serious.Here the vibrations produce corresponding iiuctuations of the localoscillator frequency and this, in turn. frequency-modulates theintermediate frequency carrier at the vibration frequency. Thisfrequency modulation component is detected by the receivers frequencydetector along With the desired component of modulation and, if thedetected signal is reproduced With sufficient volume, the feedback ofacoustic energy to the condenser may produce sustained oscillations atthe natural frequency of condenser plate vibration.

Unfortunately, the most commonly employed tuning condenser is also theone most subject to disturbance by mechanical vibration. Reference ishad, of course, to the conventional gang condenser Whose air dielectricprovides a minimum of damping for the spaced plates, thereby permittingvery appreciable vibration in response to acoustic energy issuing fromthe loudspeaker. It will be understood. of course, that many other (Cl.'i3-69) types of condensers, such as the trimmer condensers which oftensupplement the gang condenser, are also sensitive to mechanicalvibration, but since the effect is most noticeable and injurious in thegang-type tuning condenser, the

subsequent discussion will be more speciicallyv directed thereto.

Since virtually every gang condenser, no matter how carefullyconstructed, exhibits such microphonics, provided the level of acousticenergy impinging thereon be sufficiently high, it becomes imperative tocheck samples of each newly-designed condenser in an effort to determineits microphonc response. Frequent tests during the production of a givencondenser assembly are also necessary in order to insure uniformity ofthe product. In the past, tests intended to determine microphoncresponse have usually been carried out by placing the condenser undertest near a loudspeaker, driving the loudspeaker with audio frequencysignals, varying the frequency of these signals over the entire audiofrequency spectrum, and detecting'the amplitude variation with frequencyof the output signal derived from the condenser. From observation of thepeaks in this frequency characteristic, it was attempted to drawdefinite conclusions regarding the microphonc response of the condenser.Unfortunately, such data were exceedingly difficult to interpret, andeven more difficult to correlate with similar observations made on othercondensers.

It was, therefore, a matter of great diiTlculty to set up uniformstandards of performance and production.

Another important flawA in the prior-art test procedure lay in itsinability to reveal the specc element which was causing the mostpronounced microphonics. Thus, if, for example, pressure was applied bymeans of a probe to selected parts of the condenser in order to reducevibration of these parts, the capacity of the condenser Was ordinarilychanged. This necessitated reexamination of the entire audio frequencyspectrum by the time-consuming prior art method hereinbefore outlined inorder to determine whether the probing had actually reducedv themicrophonics or had merely altered their frequency. Accordingly, it wasvirtually impossible to locate those parts of the assembled condenserwhich were responsible for the occurrence of microphonics, and thereforethe tests were incapable of yielding reliablel information on how to imyprove the condenser structure.

It is, accordingly, a primary object of the invention to provideimprovedvtest apparatus for.

crophonic response of a condenser after incorpo.

ration thereof into the radio receiver with which it is to bepermanently associated.

It is a feature of the invention .that test apparatus constructed inaccordance therewith is adapted for the continuous indication ofmicrophonic response, while probing in the condenser for the source ofsuch microphonics.

To the foregoing general ends, lthere is provided; ancircuitarrangementfor operating the condenser under test asacondenser-microphone, transforming theelectrical signalsderivedtherefrom into their :acoustical equivalents, and feedingthea:acoustic signals back' to thehcondenser, therebyocausingzthecondenser under test'to vbrate, 0r oscillate, at its predominant naturalfrequencyn The `manner in: Which-the various condenser characteristicsaredetermined by means of the test apparatus hereinbeforel outlined-will become more-clearly'apparent from the following discussioniwhenaconsidered in -conjunction with the accompanying-drawing,-whereinthe single figure illustratesa preferred embodiment of condenser testapparatus constructed in*` accordance with the invention There `-isillustrated, in the ligure, -a condenser I0.which:is underntestfa testamplier II connectedthereto, any adjustable attenuator I 2, anaudimamplifier -I3andafloudspeaker I4. The elements. recited are4 the.components lof v a com-- pletely self-contained systemwhereincondenser- I0is disposed infclose physicalproximity to thefmouthzgof Yloudspeaker I4` Specifically,- in thev preferred embodiment,the. loudspeakerv is disposed Withgits-,cone opening in. agenerally11p-- wardcdirectiomga thin-grounded metal; plate. I5 14bein&..=laidacross thel open mouth thereof, where itis boltednor otherwiseretained-in place,

thegcondenservundeiztest being-placed directly on top of this plate.: Ifdesired, one of its terminals' max-pbenin `electricahcontact ywith plateI5; It will be understood that, if a large number of similar; condensersare yto be testedy in quick succession,l some suitable jigl may beprovided forizpositioning. consecutive-.f condensersn inthe same,rela-tive .positions on top Y of plate, I 5; Test amplifier I Iyisprovided. for the purpose of oper-y ati-ngjhegcondenser under test, asza.condenser microphone,A so.- ,thatfvibration of its plates-will'.

beetransformed into corresponding :electrical signals. suitable foryamplification and.4 renewed ap-` plication'yfto the-loudspeaker I4.Amplifier II comprises;4 essentiallyr an amplifyingV triode'A I6,

providedwith a suitablesource of positive plate potential B+.v The. testamplifier is provided'. withaan inputleadall which may be-.connectedtolthe unground `plate or plates of 1the. condenser` undentest. Positivepotential alsov derived from. thelpsourceB-lisgapplied.v tothese sameun.-V

grounded plates, via i .voltagea dropping resistors I8 and I9, theunidirectional potentials.developed` L thereacross being zkeptfromthegrid of. tube -VI 6 by-blckingf-condensers-20land 2|." All thecom-H ponents hereinbefore described as elements ofI the test amplifierare preferably contained within a single, electrically shieldedinclosure and the input lead Il is preferably kept as short as possible,these precautions being necessary to insure that stray capacitance willnot obscure the capacitance-variationsl ofv condenser,V I 0. Theattenuatorv I2 is connected :to theoutput 'of test amplifier I I and ispreferably calibrated in some convenient units. The output of theattenuator is applied to aconventional audio amplifier I3 and thelamplified output signals derived therefrom areA returned to speaker I4.

The entire system thus forms a closed feedbackiloop whosefoperation isas follows: Random condenser- I Il.,- Thefvariations in capacitance thusproducedwill.cause-the gr-idvoltage vof--tube AII to vary, witha'resultingsignaloutputfromtest l amplifier v.IVI lwhich is again -fedback to. loudspeakery I4, andthence via condenser I0, back to the-ytesta-amplifier. I I: If= the attenuation. of

attenuator/Itis madefsufliciently-.low and :thev

gpain-A of amplifier- I3-suiliciently high, then the grain-.around thefeedback loop will exceed unity, withv ..theresultz that the system-willbeginto oscillatefat.thepredominant natural-frequency of mechanicalvibration `of .condenser I Il; This oscillationwill manifest itself asavhowl issuing from'.

loudspeaker I4;l The intensityv of the= howlv is afunction oftheseverity .of the condensermicrophonicsyand of-.the settingvof attenuatorI 2, Since y this: ,arrangement .does-not :.depend; f in any way; upon.-the. electricalftuning effect ofY the condenser, Vitiswpreferablypractical .toA probe into the condenser-structure; atfany -point which;may be` v accessible, in an-effortto-determine .which porr tion ofthecondenser structure? isresponsible yfor themicrophonics.f The quality ofvariousicon-Y A denser-s I with @respect -to their .microphonc responsemay-be :determinedby correlating the .in-1

tensityof-.fthe-microphonics -with the setting-of the.attenuator for'any vgiven specimen. For example, one `dzest lprocedure ,which has. beenfolonf-` plate-f I5 2 in'gthe manner hereinbefore described, connectingtest amplifier I I .theretov and then {decreasing-,f` `,the attenuationof Aattenuator I2 wuntil microphonics fareaflrst observed:y in thesystem; This, valueA of .attenuation is .noted and' the attenuation thenincreased :until the L micro.-

p ilovved,inzpracticezconsistsIof placinga condenser -v phonicscease;the'. corresponding. .attenuation is f; again f.noted.:. For; avariable, condenser,` .this

process 1 is?. repeatedifor several .discrete settingsof :the tvariableelement.' Since. such' tests yield.v

results :.which' cani .be` easily -and accurately Y reproduced cnsubsequent. occasions, the two valuesy ofattenuationzcorrespondingitothe starting -and stoppingpf theimicrophonics may be taken. to

f be-lparameters indicative -of the' -microphonicresponse:qualityfoftthecondenserundertest. Thus respect .tomicrophonicsaccurately evaluated. InA

addition; a itl :haslbeem possible to determine, by

probingr twhich'fportion of the' condenser' structure -wasfresponsibleforV the occurrence of microphonics,l thereby-renderingspossiblesubstantial improvements inltheir"ba'siomechanical construe-- It Will beunderstood that it is not essential, in practicing my invention, thatthe relative disposition of loudspeaker and condenser under testillustrated in the figure be maintained.v It is, as a matter of fact,perfectly feasible to test the condenser subsequent to its installationin a radio receiver. In that case, the true operating conditions of thereceiver may be very closely approximated by connecting the testamplifier to one set of condenser plates, grounding the other set ofplates, connecting the output of the attenuator to the audio amplifierof the receiver, and using the speaker of the receiver, in its properlocation in the receiver, to produce the desired mechanical vibration.Again the attenuator setting may be varied and the conditions forstarting and stopping of microphonics observed, and the receiverarrangement rated accordingly.

It is clear that still other arrangements may occur to those skilled inthe art. I, therefore, desire the scope of my inventive concept to belimited only by the appended claims.

I claim:

1. Test apparatus for determining the microphonic response of acapacitor, said apparatus comprising: an electrical circuit adapted tobe connected to said capacitor and arranged to sense variations in thecapacitance thereof due to vibration of the capacitor elements andfurther arranged to transform these sensed variations into an electricalsignal having corresponding amplitude variations, an electronicampliiier arranged to amplify said electrical signal, a variableattenuator connected intermediatesaid electrical circuit and saidamplifier, and .a loudspeaker connected to the output of said amplierand arranged to transform into mechanical vibrations the electricalsignal output of said amplifier, said loudspeaker being adapted to bepositioned in spatial proximityto said capacitor.

2. Test apparatus for determining the microphonic response of acapacitor, said apparatus comprising: a source of predeterminedunidirectional potential, means for applying said potential across theterminals of a capacitor under test, the potential thus developed acrosssaid terminals being susceptible of variation in accordance withvariations in the capacitance of said capacitor due to mechanicaldeformation of the capacitor elements, electrical amplifying meansconnected across said terminals and arranged to amplify said voltagevariations, variable attenuating means arranged to control the amplitudeof said voltage variations, a loudspeaker adapted to be disposedproximately to said capacitor, and means for applying the voltagevariations derived from the output of said amplifying means to saidloudspeaker to produce a corresponding acoustical output therefrom.

3. Apparatus according to claim 2, characterized in that saidattenuating means has a range of said apparatus from a value above to avalue below unity for any practical capacitor.

4. Test apparatus for determining the microphonic response of acapacitor, said apparatus comprising: circuit means adapted to beconnected to a capacitor under test for developing an electrical signalin response to mechanical Vibration of the structures comprising saidcapacitor, means for amplifying said signal, means for regulating theamplitude of said amplified signal, an electromechanical transduceradapted to be disposed proximately to said capacitor, means for applyingsaid amplified signal to said electromechanical transducer to producecorresponding mechanical vibrations therein, and a vibrationtransmissivemedium adapted to be disposed intermediate said transducer and saidcapacitor whereby said produced vibrations are transferred to saidcapacitor.

5. Apparatus for determining the microphonic response of a capacitor,said apparatus comprising: a source of unidirectional potential, meansincluding an impedance for applying said Vpotential to the terminals ofa capacitor under test, a first amplifier connected to said terminalsand arranged to amplify varying electrical signals developed thereacrossin response to capacitance changes due to mechanical deformation of thestructures comprising said capacitor, a variable attenuator connected tothe output terminals of said rst amplifier, a second amplier connectedto the output terminals of said attenuator, and a loudspeaker connectedto and driven by the signal derived from said second amplifier, saidloudspeaker being disposed in such proximate spatial relation to saidcapacitor as to bringv said capacitor substantially within the path ofacoustical energy issuing from said loudspeaker.

6. Apparatus according to claim 5 characterized in that the range ofattenuation provided by said variable attenuator is suiiiciently wide topermit variation of the overall gain of the apparatus from a value belowunity to a value above unity for any practical capacitor.

7. Apparatus according to claim 5 characterized in that said means forapplying said potential to said capacitor terminals and said firstamplifier are -capacitively shielded.

GEORGE O. SMITH.

REFERENCES CITED The following references are of record in the

